Security, code quality and frontend improvements

- Move SECRET_KEY out of docker-compose into .env (env_file), add .env.example
- Add flask-limiter with 10 req/min on login route; introduce util/limiter.py
- Replace direct mysql.connector.connect() calls with MySQLConnectionPool via util/db_pool.py
- Fix deprecated datetime.utcnow() -> datetime.now(timezone.utc) in auth/login.py
- Remove dead /api/scripts 410 route from admin.py
- Add MD5 security warning in Md5Tool.jsx
- Add ErrorBoundary component and wrap App.jsx
- Expand README with setup guide, screenshot and project structure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app.py

@@ -8,14 +8,15 @@ from flask import Flask, send_from_directory, redirect
 from util.logger import logger
 from util.db_config import is_configured, load_config, test_connection
 from util.setup_routes import setup_blueprint
+from util.limiter import limiter
 from auth import auth_bp
 from tools import md5_blueprint
 from admin import admin_bp
 
 app = Flask(__name__, template_folder="templates")
+limiter.init_app(app)
 
-
-# 📦 Blueprints registrieren
+# Blueprints registrieren
 app.register_blueprint(setup_blueprint)
 app.register_blueprint(auth_bp)
 app.register_blueprint(md5_blueprint)